Internal combustion engines are subjected to a variety of external as well as internal variable conditions ultimately affecting the performance of the engine. Examples of such conditions are engine load, ambient air pressure and temperature, timing, power output and the type and amount of fuel being consumed. In most cases fuel is pumped from a source by way of a low pressure rotary pump or gear pump to high pressure pumps, known as unit injectors and associated with corresponding engine cylinders. Such unit injectors conventionally include a positive displacement piston driven by a cam which is mounted on an engine driven camshaft. Recent and upcoming legislation resulting from a concern to improve fuel economy and reduce emissions continues to place strict emissions standards on engine manufacturers. In order for new engines to meet these standards, it is necessary to produce fuel injectors capable of achieving higher injection pressures and shorter injection durations. This high pressure, fast system response requirement is determined primarily by the opening and closing response of the valves internal to the injector and the amount of fuel under compression. In addition, although unit injectors of the electrical, mechanical, hydraulic or electromechanical types are known, systems employing such injectors often lack reliable control of injection from cycle to cycle.
One well known approach for providing cycle by cycle control is to employ a solenoid valve in combination with the unit injector to vary the quantity and timing of fuel injection during each cycle. For example, in U.S. Pat. Nos. 4,129,253 to Bader et al. and 4,392,612 to Deckard et al., an electromagnetic unit fuel injector is disclosed including a single, cam-operated injector plunger, a solenoid controlled valve for determining the beginning and ending of injection and thus the timing and quantity of fuel injected during each cycle of plunger movement. The solenoid controlled valve operates to allow fuel to flow into and out of the pumping/metering chamber of the unit injector when open but traps fuel in the chamber when closed to cause the unit injector plunger to force fuel though the injector nozzle into an associated combustion chamber of the engine. A tip-mounted valve may also be provided for resisting blow back of exhaust gas into the pumping/metering chamber of the injector while allowing fuel to be injected into the cylinder. Accordingly, injectors of the type disclosed employ both a solenoid operated valve as well as a tip-mounted valve. With this construction, the solenoid controlled valve is normally biased into open condition while the tip valve is normally biased into a closed position, thereby allowing excess fuel to be discharged from the pumping/metering chamber through a drain passage. Upon movement of the solenoid operated valve to a closed condition, a sufficient pressure will build up so as to displace the tip-mounted valve and allow the injection of fuel to commence.
Both Bader et al. '253 and Deckard et al. '612 disclose fuel injectors having a solenoid controlled valve positioned offset from the central axis of the fuel injector body. This configuration creates a wide and bulky injector body resulting in a loss of space available for engine intake and exhaust valves, thereby eliminating the potential use of this design on many engines.
U.S. Pat. No. 4,482,094 issued to Knape and U.S. Pat. No. 4,741,478 issued to Teerman et al. both disclose fuel injectors having solenoid actuators mounted coaxially with the injector body thereby inherently providing a fuel injector body having a relatively smaller radial extent. However, in the Knape design the coil of the solenoid is arranged concentrically around the injector plunger. Therefore, the solenoid coil inner diameter is determined by the diameter of the injector plunger. As a result, the solenoid coil and fuel injector body have an unnecessarily large diameter. Again, this design results in the loss of space available to the engine intake and exhaust valves. In addition, the armature/control valve arrangement utilizes the magnetic lines of force of the outer pole of the stator positioned beyond the outer radial extent of the coil. Since the armature must be positioned closely adjacent these outer poles to generate the force requirements of the valve, the armature is required to be larger than the outside diameter of the coil. This large armature mass increases the effects of inertia thereby undesirably increasing response time. The Teerman et al. reference discloses a unit fuel injector wherein the solenoid actuator is positioned axially between the injector plunger and the normally closed nozzle tip valve. However, although decreasing the outer diameter of the injector body, this axial arrangement creates a fuel injector body having an undesirably large axial length. In addition, the Teerman injector includes a solenoid actuator which utilizes the magnetic lines of force adjacent the outer pole requiring a relatively large armature thereby causing a decrease in response time.
Mother disadvantage of the prior art discussed above is that the valve seat of the solenoid operated control valve is positioned a relatively large distance from the pumping/metering cheer. This arrangement increases the length of the fuel transfer passages thereby increasing the compressed fuel volume and, consequently, the response time.
As noted above, injectors of the type disclosed employ both a solenoid valve as well as a tip-mounted valve so that upon movement of the solenoid valve to a closed position, a sufficient pressure will build up to displace the tip-mounted valve and allow the injection of fuel to commence. However, a delay between the closing of the solenoid operated valve and the opening of the tip valve can occur, and such delay can be dependent upon internal and external variables which affect engine operation. Therefore, this delay may be somewhat indeterminate and may cause a variation in the timing and/or amount of fuel injected resulting in a degradation of the engine's ultimate performance.
Solenoid control of unit injectors provides important advantages, not the least of which is the ability to use computer generated control signals. However, solenoid operated injectors of the type known up to the present, and discussed above, have been costly to manufacture. A large component of this manufacturing cost is due to the solenoid operated valve itself which must operate reliably at high speed over many millions of open-close operating cycles. Previously known unit injector designs have often accentuated the operating demand on the solenoid valve by requiring the valve to operate against high injection pressure (for example, around 15,000 psi) to obtain proper fuel burning characteristics. Strong electromagnetic forces developed in a very short time are required when the valve must move against such high pressures.
The fuel injector disclosed in U.S. Pat. No. 3,861,644, to Knape, as discussed above, includes a solenoid operated valve wherein the electromagnetic force must overcome the fluid pressure force so as to properly operate the valve. Also, this valve is for use in a system similar to those set forth above which employ a second, tip-mounted valve for proper injection. In this regard, the patent to Bader, Jr. et al (U.S. Pat. No. 4,129,253) also discloses an injector including a solenoid operated valve must be held closed against injection pressure.
In U.S. Pat. No. 4,275,693, issued to Leckie, a fuel injection timing and control device is disclosed wherein injection of fuel which is pressurized by way of a plunger/piston arrangement is carried out by the use of a solenoid controlled sleeve tip valve, wherein the solenoid is mounted coaxially with the central axis of the injector body. Here, however, fuel is continuously maintained under pressure by way of check valves and an accumulation chamber and when the solenoid is activated, the valve allows a metered portion of the fuel to be injected through discharge passages. In one embodiment (illustrated in FIG. 2) a second solenoid is provided to move the pin-type tip valve to its closed position. Leckie specifically teaches the use of a sleeve valve in combination with fuel ports oriented perpendicularly relative to the direction of opening and closing of the sleeve. By this arrangement, Leckie asserts that the fuel within the injector exerts no opening or closing force on the valve but no provision is made for fuel pressure variations across the diametric cross-section of the valve element as the valve element moves between its open and closed positions. The relatively large sealing diameter of a fuel injector control valve, as illustrated, for example, in the Knape '094 and Teerman et al '478 patents, makes the valve sensitive to component tolerances, wear and erosion.
Consequently, the prior art fails to disclose a unit fuel injector having a solenoid operated valve arranged to minimize both the width and axial length of the fuel injector body. Also, there is a need for an electromagnetic unit fuel injector which minimizes system response time by reducing the armature mass while minimizing the volume of compressed fuel. In addition, the prior art fails to disclose a solenoid operated tip valve for a fuel injector wherein the forces on the tip valve are sufficiently balanced to permit the use of a practical, highly compact, high speed solenoid operator.